Quantity measurement of bulk materials is a complex issue in the truest sense of the word. Companies search in vain for a universal approach. So which weighing technology is the right one?
It is important to find the right solution for the respective production process. This guide is designed to help you do just that.
Regardless of whether it is a continuous scale or a belt scale, each system has its strengths and weaknesses. And these need to be compared.
This is the only way to find the right device and strategically optimize your production processes.
You know your production best. But do you also recognize its potential?
We would like to show you the possibilities and limitations of different weighing systems. So that you can find the optimum solution for your workflow.
The most important selection criteria for the ideal measuring and weighing technology
Do you want to know which system is best suited to you and your company? Then you should take a closer look at the following three criteria:
Product features
It is not only important what you process, but also the material attributes at the time of measurement. The following factors are decisive:
- Bulk density
- grain size
- temperature
- flowability
- compressibility
- Wear properties
- Ex-classification
Production environment
It is not only the individual elements of the production chain that play a role here, but also where exactly in the production process the measurement should take place.
The following aspects are important for the corresponding realization:
- Location of the installation (in or after a conveyor line, pneumatic conveyor line, free fall)
- Type of conveyor (belt conveyor, Redler, screw, vibro-channel, pneumatic)
- Type of feed (via conveyor system, from silo, via rotary valve)
- Special features (dust-tight housing, dust extraction, explosion protection)
Technical requirements
The range of functions and configuration depend first and foremost on the respective production process. Decisive influencing factors here are
- Area of application (billing, pre-loading, control, monitoring)
- Measuring accuracy and reproducibility
- Throughput rate including turndown ratio
- Running time (operating time)
- Calibration and maintenance costs
Proven measuring methods for various bulk materials
The range of weighing systems is enormous. Different measuring methods can be used depending on the material and production process.
In the area of throughput measurement of bulk goods, belt scales in various designs are predominantly used.
However, continuous scales have also proven themselves in many places.
In the following, we would like to take a closer look at the possibilities and limitations of different weighing technology. And thus shed some light on the selection process.
Weighing technology: built-in belt scales
Belt scales of this type are primarily used in belt conveyors.
Bulk material flows can be monitored and recorded and the measured values evaluated accordingly.
belt scale for belt conveyors with one weighing idler station
belt scale installed in belt conveyor with one weighing idler station
Weighing controller for belt scales for mass flow rate measurement
Measuring principle
The conveyor belt of the system runs over the measuring roller frame integrated in the weighing frame with one or more high-precision load cells.
During calibration, the belt's own weight is defined as the zero or standard value.
The weight signal can then be multiplied or integrated:
By multiplying the basic weight by the belt speed, the conveying capacity can be determined in tons per hour.
The absolute quantity, on the other hand, results from the integration of the current measured value.
The measuring accuracy can be further optimized by installing several roller chairs.
Depending on the belt speed, the lower measuring range limit is between three and ten tons per hour.
Measuring accuracy
For conveying capacities in the upper measuring range of more than 30 tons per hour, a measuring accuracy of one to two percent is realistic.
The measuring error can be reduced to 0.5 percent with the multi-roller systems mentioned above.
However, this configuration requires regular taring of the belt weight and cyclical recalibration by reference weighing.
Areas of application
In-belt scales are particularly suitable for recording and controlling large product flows in the processing industry.
Examples include quarries, gravel and coal plants as well as the cement, ore and animal feed industries.
This technology has also proven itself in the loading of transport vehicles such as trucks, freight cars and ships.
Advantages
In addition to their low acquisition costs, built-in belt scales are also considered an extremely robust solution with a long service life.
In addition, the devices are largely independent of the respective product properties and can be easily integrated into existing conveyor systems.
Even calibratable versions are available.
Disadvantages
Due to their dependence on the changing belt properties, weighing systems of this type are only suitable for use at low conveying capacities to a limited extent.
Variations in temperature and belt tension as well as wear can have a decisive influence on the measuring accuracy.
With calibratable devices, these influencing factors can be minimized by a so-called gravimetric belt tensioning station.
Weighing technology: Belt feeders
These belt scales are also primarily used to measure the quantity of bulk material in conjunction with belt conveyors.
Measuring principle
To a certain extent, the mode of operation is identical to that of a built-in belt weigher.
However, with weighfeeders, the conveyor can be ideally adapted to the product and production.
Tailor-made belts, constant belt tension and additional scrapers ensure optimum measuring accuracy.
Another special feature is the adjustment of the conveyor speed to the throughput rate.
This allows the belt speed to be reduced to 0.02 m/ per hour for low throughput rates.
Measuring accuracy
In the output range between 20 and 100 percent, any measuring errors vary between 0.25 and 2 percent.
Regular taring and recalibration by reference weighing is required.
Areas of application
Belt weighfeeders are suitable for recording and controlling medium to small product flows in almost all industrial sectors.
complete weigh feeder
Advantages
The combination of conveyor and weighing system as well as the adaptation to product characteristics and production requirements enables measurement results close to the optimum.
Calibratable versions are also available here.
Disadvantages
Production according to individual customer specifications is associated with a comparatively high purchase price.
In addition, adjustment and maintenance can be relatively time-consuming.
Weighing technology: Optical belt scales
Scales of this type are primarily used to measure and control the volume flow of mobile and stationary conveyor belts.
Where gravimetric belt scales reach their limits, optical belt scales are the logical alternative.
optical belt scale for rock installed in belt conveyor, volume and mass flow rate measurement
Measuring principle
A laser scanner and sensor head are used to record the load profile without contact at the conveyor discharge and transmit it wirelessly to a computer unit.
The material and layer height determine the measured value, which is multiplied by the conveyor speed to give the respective volume flow.
Specific conversion factors can also be used to determine the corresponding mass flow.
Measuring accuracy
With 3,200 measuring points per second, the measuring tolerance is plus minus one millimeter.
Areas of application
Optical belt scales are primarily used in mobile screening and crushing plants and shredders, but also for measuring the volume of tunnel bores.
Advantages
The versatility of this weighing technology is exceptional.
Regardless of the belt tension, even particularly light bulk materials can be measured precisely on comparatively short belt conveyors.
Installation and commissioning are also completed quickly.
Disadvantages
The optical sensors result in comparatively high acquisition costs and require periodic cleaning in the event of heavy dust formation.
Depending on the bulk material and bulk density, the conversion of volume and mass is only partially reliable.
Weighing technology: Impact plate scales
Baffle plate scales have proven to be a simple and robust method of recording mass flow in dust-tight conveying paths.
In the production process, the bulk material scales can be ideally combined with screws, vibrating chutes, conveyor belts and rotary feeders.
Measuring principle
Inside the measuring device, the bulk flow is directed onto the inclined impact plate by means of a chute or inlet pipe.
The impact force generates a mechanical deflection, which is converted into an electrical signal by means of a sensor.
The integrated evaluation electronics processes the measured values accordingly and reads them out.
impact flowmeter for bulk materials, installed in pipework, mass flow rate, total mass measurement
measuring principle impact scale
Measuring accuracy
With constant product properties, any measurement errors are in the range of two to three percent. The turndown ratio is one to three.
Areas of application
Impact plate scales can be used to measure, monitor and control free-flowing bulk materials in almost all branches of industry.
Both small and large material flows can be reliably processed.
Advantages
They are a robust and cost-effective solution. In addition, scales of this type are largely maintenance-free.
Disadvantages
In production chains with fluidized products, impact plate scales are only suitable to a limited extent.
In addition, the measuring accuracy is highly dependent on factors such as particle size, elasticity and bulk density.
Weighing technology: Continuous scales
Centripetal force scales are an alternative for precise mass flow measurement of flowing bulk materials.
The conveyed material can be fed via almost all common conveyors.
Find out more about this type of bulk material measurement in this external article.
Measuring principle
The physical basis for weight measurement is Newton's second law of classical mechanics.
According to this law, the centripetal force is the product of mass and acceleration.
In the production process, powder, granulate or other products are systematically fed into a strain gauge load cell via a circular chute.
The measured centripetal force is directly proportional to the mass flow.
The load cell signal is standardized by the downstream electronics, whereby vibration-related interference is filtered out at the same time.
Both the absolute quantity and the current throughput quantity in kilograms per hour can be read off via the evaluation unit.
flowmeter for plastic pellets, scale installed behind rotary valve, below silo, mass flow rate, total mass measurement
measuring principle of flow measurement for bulk materials, centripetal force scale
Measuring accuracy
The potential measurement deviation is between 0.2 and 0.5 percent, with a turndown of one in ten.
High reproducibility is also guaranteed in the low throughput range.
Areas of application
Centripetal force scales are predestined for the precise measurement and control of hourly throughput quantities from 30 kilograms to 300 tons.
The dosing of batches and the loading of silo vehicles can thus be optimized and processes can also be monitored in real time.
Advantages
In contrast to impact plate scales, factors such as bulk density, particle size or elasticity have at most a marginal influence on the measurement results with centripetal scales.
Even with pulsating flow, the precision with regard to free-flowing bulk materials is exceptional.
Thanks to the integrated cleaning by air nozzles, no maintenance routine is required.
The scales are also ideal for bulk materials with a low density.
Disadvantages
Scales of this type are only suitable for products with good flow properties.
Weighing technology: Screw scales
Where other scales quickly reach their limits with powdery bulk products such as white lime, hydrated lime or cement, dust-tight weighing screws are the reliable alternative.
With the necessary weighing technology know-how, reliable throughput measurement can be achieved with screw conveyors.
Measuring principle
On the drive side, a maintenance-free joint ensures that the screw is suspended, while the weighing process on the discharge side is carried out via a precise tension or compression force measuring cell.
At the same time, an incremental encoder mounted on the front side records the transport speed in proportion to the rotational speed.
After professional calibration, the product of net weight and speed can be controlled and regulated as an hourly conveying capacity in kilograms or tons.
A clean decoupling of the inlet and outlet is a prerequisite for optimum transportation of the bulk material and precise measurement results.
Controlling the conveying speed is also essential.
Measuring accuracy
With regular taring and products with low adhesive properties, potential measuring errors are in the range of two to three percent.
Areas of application
Weighing screws guarantee dust-free measurement, control and monitoring of small and medium-sized product flows.
Advantages
The characteristic design enables the simultaneous conveying and weighing of powder-fine products in a closed, dust-free system.
Maintenance costs are extremely low due to the dust-tight design.
Existing screw conveyors can be easily upgraded with weighing components and complete solutions are also possible.
Disadvantages
The basic requirement is the controlled feeding of the bulk material via a rotary valve or an additional screw.
In addition, screw scales are only suitable to a limited extent in production processes with non-free-flowing material.
Weighing technology: FlowCube
The measuring device for combined density and flow measurement in vertical downpipes:
Bulk material in the form of granulate or powder can be reliably measured and dosed with manageable effort.
A closed, vertical measuring body including a maintenance-free control valve and a high-precision load cell are installed in a dust-tight housing with inlet and outlet nozzles.
3D-model of improved impact scale with bulk density measurement
Measuring principle
The bulk density is first determined using the volume of a vertical measuring body.
The flow rate is then calculated and monitored based on the actual increase in mass.
Both the density in kilograms per liter and the flow rate in kilograms per hour can be read out simultaneously via the display of the evaluation unit.
Areas of application
FlowCubes are suitable for measuring and controlling throughput quantities in the range of ten kilograms to twenty tons per hour.
Depending on the manufacturing process, dosing and production can also be controlled and monitored.
Advantages
The acquisition costs for the compact, dust-tight system are comparatively low.
Self-calibration means that periodic taring or comparative weighing is not necessary and maintenance costs are also minimal.
Disadvantages
A continuous flow is required. FlowCubes are only designed for sudden changes in measured values to a limited extent.
Conclusion
There is the right measurement technology for almost every bulk material.
Optimal measuring methods enable the strategic optimization of the entire production process.
With this guide, you will hopefully be one step closer to the right solution for your production process.
With personal advice, it is almost certain to impress with its reliability and sustainability.